{bp-15159} fs/shm/shmfs_alloc.c: Allocate zero-initialized memory in flat build

[jerpelea]

Summary

POSIX requires that the shm objects are zero-initialized. This has been broken in some earlier commits (starting from 9af5fc5)

Also fix the flat build memory allocation to allocate both object data and payload in the same chunk (as the comment also suggests). This saves allocations and memory in a system with lots of shm objects.

This PR fixes errors in applications, which run in "CONFIG_BUILD_FLAT", and assume that newly allocated shared memory is zero-initialized.

This also reduces memory consumption in smaller devices, when each shared memory object is allocated in a single chunk as originally.

The errors were introduced earlier when the SHM allocations were cache-line aligned.

NOTE
included
#15167

Impact

RELEASE

Testing

CI

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR appears to meet the NuttX requirements. It clearly outlines the problem (POSIX violation and inefficient memory usage), the solution (zero-initialization and consolidated memory allocation), and the impact (fixes application errors, reduces memory consumption). The testing section, while brief, indicates CI testing was performed, which is acceptable if the CI system is robust. The explicit mention of "RELEASE" under Impact suggests this is a significant change potentially affecting users, so more detailed testing information might be beneficial if available. Referencing the related PR is also helpful for context.