linux-sandbox: Retry remount-ro on EBUSY (https://github.com/bazelbuild/bazel/pull/29433) ### Description MakeFilesystemMostlyReadOnly issues MS_BIND|MS_REMOUNT|MS_RDONLY for every entry in /proc/self/mounts. On busy mounts (notably "/") this races with concurrent symlink path lookups: pick_link() in fs/namei.c calls touch_atime(), which holds the per-mount writer counter via mnt_get_write_access() across the inode update. mnt_hold_writers() in the remount path returns -EBUSY if the counter is non-zero, causing the sandbox setup to die. The race window is short (a single update_time call), so a brief backoff-and-retry resolves it cleanly. Use the same constants runc has shipped since 2014 (5 attempts, 100 ms apart). This is my proposal for https://github.com/bazelbuild/bazel/issues/29424. ### Motivation Fixes https://github.com/bazelbuild/bazel/issues/29424. ### Build API Changes No ### Checklist - [x] I have added tests for the new use cases (if any). - [x] I have updated the documentation (if applicable). ### Release Notes RELNOTES: None Closes #29433. PiperOrigin-RevId: 922094350 Change-Id: I7ff8b1b5ad7992f164b44a6742c4f70cadb18f00
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